The present invention relates to new and improved refractories, to electrically heated furnaces containing the same, and to methods of making the improved refractories.
Refractories are used for lining the internal heated chambers of furnaces that are used to heat materials, and in many instances the refractories must have particular properties to withstand the reactivity of the materials being heated and/or the furnace atmosphere. Problems exist, for example, in electrical furnaces that are used to melt glass, because the glass has a fluxing action on many types of refractories, and because the refractories which are resistant to the fluxing action have an electrical conductivity that is greater than that of the molten glass. Similar problems exist in other types of furnaces, so that in many instances there is a need for refractories which have a higher electrical resistance than do the prior art refractories. In still other instances, refractories are used as the electrodes which contact the molten glass, and in these instances, a need exist for increasing the electrical conductivity of the refractory of the electrodes so as to prolong their life. Still other applications exist where a benefit can be derived by increasing or decreasing the electrical conductivity of the refractories used therein.
According to the present invention, it has been discovered that the electrical conductivity of a refractory can be increased or decreased without appreciably effecting the ability of the refractory material to withstand attack by the materials which the refractory contains. As for example, the conductivity of pure chromic oxide at the fusion temperature of molten glass is greater than that of the molten glass, and it has been discovered that the lattice structure of the chromic oxide refractory can be changed to cause the chromic oxide to have an appreciably greater electrical resistance to thereby decrease the power drain or loss of electrical energy from an electrically heated glass melting furnace. This is accomplished by diffusing a dopant into the lattice structure of essentially pure particles of chromic oxide to decrease the amount of electron carriers in the lattice structure that is available for carrying electric current.
According to the invention it has been discovered that refractories, which at room temperature behave as insulators, exhibit appreciable semi-conductivity at the elevated use temperatures of the furnaces wherein they are installed.
If a dopant is used having an atomic radius of approximately the same atomic radius as the metal atoms in the crystal lattice of the refractory particles, it can diffuse into the lattice without forming deposits at the grain boundaries between the particles. By controlling the purity of the lattice structure so that it only contains the dopant desired, and by eliminating fluxing materials at the grain boundaries, a body is produced having changed electrical conductivity at elevated temperatures. It has been found, for example, that when a mixture of basically pure particles of chromic oxide and a small amount of titanium oxide are sintered, that the electrical resistivity of the resulting refractory is appreciably increased so that electrical furnaces for melting glass now are a commercial possibility. It has been found, that the amount of the dopant that is used must not be appreciably in excess of that which will diffuse into the crystalline lattice of the refractory particles, and that this amount generally corresponds to the solubility of the dopant in the refractory particles at the use temperature.
Most of the prior art refractories with which I am aware have used sintering aids, sometimes called shrinking agents, or fluxing materials, to either lower the sintering temperature, or increase the density of the refractory produced. According to the invention it has been found that these sintering aids are not only detrimental, but that they prevent the synergistic effect of the present invention from taking place. The effect of impurities, therefore, with respect to the materials of the present invention is critical, and the amounts of undesired impurities must be held within very narrow limits.
Because it has been discovered that refractories are semi-conductors at their elevated use temperature, the following theory has proven beneficial. Electrical conductivity of p-type semi-conductors occurs by reason of defects in the metal atoms of the lattice, and in the case of chromic oxide refractory, these defects are negative triple ionized chromium vacancies and will hereinafter be designated V.sub.Cr '". Since oxygen is the other lattice material, the oxygen in the lattice will be controlled by the crystal-vapor equilibrium wherein: EQU 3/2 O.sub.2 .revreaction. 2(V.sub.Cr '") + 3O.sub.o + 6h
where:
O.sub.o represents oxygen at an oxygen site; and PA1 h represents positive holes. PA1 p is the concentration of positive holes; and PA1 [] denotes concentration of the defect. PA1 n is the concentration of electrons; PA1 [V.sub.o.sup.++ ] is the concentration of positive double ionized oxygen vacancies; and PA1 [Ti.sub.Cr.sup.+ ] is the concentration of the dopant, Ti, positively charged located on a Cr site. PA1 n is the concentration of electrons; PA1 [Mg.sub.Cr '] is the concentration of the dopant Mg negatively charged located in Cr sites; and PA1 [V.sub.o.sup.++ ] is the concentration of positive double ionized oxygen vacancies. PA1 e represents electrons. PA1 n is the concentration of electrons; PA1 [V.sub.o.sup.++ ] is the concentration of positive double ionized oxygen vacancies; and PA1 for constant P.sub.o.sbsb.2 EQU K' = [V.sub.o.sup.++ ]n.sup.2
The equilibrium constant for the above equation, therefore, becomes: ##EQU1## where: P.sub.o.sbsb.2 is the partial pressure of oxygen;
For constant p.sub.o.sbsb.2, then k' = [V.sub.Cr '"].sup.2 p.sup.6. When the pure Cr.sub.2 O.sub.3 lattice is doped with a foreign atom of higher valence than Cr.sup.+++, for example, T.sub.1.sup.++++, the electrical neutrality condition is given by: EQU n + 3[V.sub.Cr '"] = p + 2[V.sub.o.sup.++] + [Ti.sub.Cr.sup.+ ]
where:
Employing Browers Method of Approximation for the electrical neutrality condition, the modified equation becomes: ##EQU2##
It can readily be seen that as the concentration of the dopant [Ti.sub.Cr.sup.+ ] increases, p must decrease in order to maintain a constant K'. Since the electrical conductivity is directly proportional to p, the electrical conductivity of p-type Cr.sub.2 O.sub.3 refractory is decreased when dopant additions like Ti.sup.++++ exhibiting a higher valence than the base metal Cr.sup.+++ are used.
What has been said for titanium will occur with any material having a higher valence than Cr.sup.+++ provided that these materials have an atomic radius sufficiently similar to that of the chromium so that it can diffuse into the chromic oxide lattice. What is more, the above theory will hold true for any p-type semi-conductor material. In the case of chromium, therefore, the following materials can be used as dopants to increase the electrical resistance of the chromic oxide refractory: titanium, zirconium, hafnium, vanadium, niobium, tantalum, molybdenum, wolfram, ruthenium, osmium, iridium, silicon, germanium, and tin.
For the case where it is desired to increase the conductivity of a p-type refractory, the following theory applies. The electrical neutrality condition is satisified when: EQU n + 3[V.sub.Cr '"] + [Mg.sub.Cr '] = p + 2[V.sub.o.sup.++ ]
where:
By using Brower's Method of Approximation: ##EQU3##
This equation shows that as the dopant concentration increases, p (the positive holes) must increase in order to maintain a constant K'. The electrical conductivity of all p-type refractories, therefore, is increased with additions of dopants exhibiting a lower valance than the metal of the lattice.
In n-type refractories electrical conduction occurs principally by reason of ionic defects in the oxygen or negative element of the refractory lattice. The predominant ionic defect, therefore, will be designated V.sub.o.sup.++. The crystal equilibrium equation is: EQU 1/2 O.sub.2 + V.sub.o.sup.++ + 2e' .revreaction. O.sub.o
where:
The equilibrium constant for the above equation is: EQU K = ]V.sub.o.sup.++ ]n.sup.2 p.sub.o.sbsb.2.sup.1/2
where:
Let X be the positive element (+3 valance) of the lattice, and let Mg be the dopant. The electrical neutrality condition is satisfied when: EQU n + 3[V.sub.x '"] + [Mg.sub.x '] = p + 2[V.sub.o.sup.++ ]
By Brower's Method of Approximation: ##EQU4##
This equation shows that as the dopant concentration increases, n must decrease in order to maintain a constant K'. For n-type refractories, therefore, the electrical conductivity decreases with additions of dopants, such as Mg.sup.++, exhibiting a lower valance than the metal of the refractory lattice.
When n-type refractories are doped with materials, designated Ti.sup.++++, such as titanium, having a valance greater than the metal "X" of the refractory lattice, the equation for the electrical neutrality condition is: EQU n + 3[V.sub.x '"] = p + 2[V.sub.o.sup.++ ] + [Ti.sub.x.sup.+ ]
By Brower's Method of Approximation: ##EQU5##
This equation shows that as the dopant concentration increases, n must decrease in order to maintain a constant K'. For n-type refractories, therefore, the electrical conductivity increases with additions of dopants, such as Ti.sup.++++ which exhibit a higher valance than the metal of the refractory lattice.
The present invention has confirmed that refractories do in fact operate, at least qualitatively, in accordance with the above theory, and that it is possible to either increase the electrical conductivity, or decrease the electrical conductivity, of either p-type refractories or n-type refractories. This is believed to be a highly significant discovery having wide application in the refractory art.